This invention relates to electro-optical elements containing a photoconductive layer and a liquid crystalline layer, and more particularly to improving the sensitivity of such electro-optical elements by including an insulating layer in contact with the free side of said photoconductive layer.
As described in U.S. Pat. No. 3,592,527, it has been found that useful visual display devices can be produced which contain as a visual image forming element thereof a multilayer structure composed of a layer or film of a liquid crystalline material overlying a layer of a photoconductive composition.
The photoconductive properties of conventional photoconductive materials and layers containing photoconductive materials have been described in detail in the literature, for example, Schaffert, Electrophotography, published by Focal Press Limited, 1965. One property of photoconductors is the creation of primary current under the influence of an applied field when the photoconductive material is struck by photons of light. Heretofore, it has been generally felt necessary that ohmic contact, i.e., a reservoir of charge at a metal-photoconductor interface, must be present in order to obtain a quantum efficiency greater than 1 wherein photoconductive current greater than the primary current (commonly called gain photocurrent) is obtained. Further, it has been generally felt that with a blocking contact to the photoconductor, e.g., a non-ohmic contact with a Schottky barrier at the metal-photoconductor interface, photoconductive gains greater than unity could not be achieved. See, for example, "Photoconductive Gain Greater than Unity in Cadmium Selenide Films with Schottky Barriers at the Contacts," R. R. Mehta and B. S. Sharma, J. Appl. Phys., 44, 1, January, 1973
According to this article, the authors were able to achieve a photoconductive gain greater than unity with gold electrodes in contact with the photoconductor wherein the gold contacts were determined to be non-ohmic with a Schottky energy barrier between the electrode and the photoconductor and wherein the radiation utilized was bandgap radiation of the photoconductor. No insulating layer was deliberately inserted between the gold electrode and photoconductor.
Conduction through a physical, electrically insulating barrier is reported in "Thermally Assisted Tunneling in Dielectric Films," G. G. Roberts and J. I. Polanco, Phys. Stat. Sol. (a), 1, 409 (1970). In the latter article, the authors reported findings in the characteristic relationship between current flow in, and voltage applied to, an insulating organic layer sandwiched between two electrodes. No photoconductive layer is utilized and no mention of gain photocurrent.
Conduction through a semi-conductor layer adjacent a few-atoms-thin layers of insulating materials is theoretically presented in "The Physical Review B," F. Schmidlin, 1, 4, pages 1583-1587 (1970).
U.S. Pat. No. 3,732,429 discloses the use of an inorganic insulating layer in contact with a photoconductor in order to obtain a higher dark impedance in conjunction with a liquid crystalline layer. All three layers are sandwiched between electrodes.
In new and growing areas of technology, new methods, apparatus, compositions and articles of manufacture are often discovered for the application of the new technology in a new mode. The present invention relates to a new and advantageous method for providing increased current flow from a photoconductor into and through a liquid crystalline layer thereby allowing the utilization of less light in optically imaging electro-optic devices containing a photoconductive layer in contact with the liquid crystalline layer wherein the liquid crystalline layer undergoes electro-optical changes due to current or charge flow. That is, the optical sensitivity of such systems is increased.